Androgen deprivation therapy (ADT) produces remission in advanced prostate cancer (CaP). However, CaP invariably recurs as castration-resistant/recurrent CaP (CRPC) and causes death. Tissue levels of testosterone (T) and dihydrotestosterone (DHT), which is the most potent androgen, decrease acutely after ADT. Once tissue homeostasis occurs, CRPC tissue T levels remain similar to androgen-stimulated CaP and benign prostate due to intratumoral androgen biosynthesis. Tissue levels of DHT, though reduced, remain sufficient to activate the androgen receptor (AR). The inhibition of enzymes critical in the final steps in T and DHT synthesis via backdoor pathways may reduce androgen levels more effectively than ADT. In the primary backdoor pathway to DHT synthesis, adrenal progestagens are 5?-reduced and converge on 5?-androstane-3?,17?-diol (DIOL), a direct precursor that is converted to DHT by at least 4 enzymes. A secondary backdoor pathway centers on androstenedione (ASD), an important adrenal androgen, which is 5?-reduced to androstanedione. Androstanedione is further metabolized to DHT by an enzyme that also converts ASD directly to T. All 5 of the key 3?-oxidoreductases in the primary and secondary pathways share a common catalytic site but otherwise are structurally unrelated. The goal of the proposed studies is to identify and inhibit the enzymes immediately proximal to intratumoral T and DHT synthesis. Our approach contrasts to inhibition of earlier enzymes, such as CYP17A1, that lie distant from the final steps in T and DHT synthesis, with agents such as abiraterone that have extended overall CRPC patient survival by only several months. The central hypothesis is that a small-molecule inhibitor of the catalytic site shared by the five 3?-oxidoreductases will decrease T and DHT metabolism through the frontdoor and backdoor pathways. The hypothesis will be tested in 3 specific aims: Aim 1 Identify a candidate inhibitor against the catalytic site shared by the five 3?-oxidoreductases; Aim 2 Synthesize and test re-designed candidate inhibitors and conduct PK/PD and toxicity studies to produce a lead compound inhibitor of the five 3?-oxidoreductases; and Aim 3 Determine whether the inhibitor of the 3?-oxidoreductases decreases tissue T and DHT levels and impairs CRPC growth. Successful completion of the proposed studies should produce an inhibitor of the five 3?-oxidoreductases that is ready for final preclinical testing prior to clinical evaluation as a bettr abiraterone to reduce the death rate from advanced CaP.